Add gcc6 aport

Based on the last version from Alpine, but installed to /usr/gcc6 and with pretty much everything disabled except for the C compiler.

We can probably throw out a lot of patches, and make the APKBUILD much smaller. But let's focus on making it possible to compile old kernels again first. There's still a lot to do for that, see #103 (closed).

Merging as soon as CI is through with this. I've tested this locally and the GCC worked there.